1. Field of the Invention
This invention relates to a thermal transfer ribbon for printing high density/high resolution bar codes for autoidentification.
2. Description of the Prior Art
Thermal transfer ribbons for printing black and colored images comprise a support substrate, ink-side coated layer(s) and a backcoat. Thermal transfer ribbons are used in tag and label applications to image various bar codes, human readable text and company logos. The printed tags and labels are usually comprised of low density/low resolution bar codes and images.
Recently, however, because of the availability of high resolution thermal print heads (200 to 400 dpi, i.e, heating elements per linear inch), end users of thermal transfer ribbons are now printing bar codes with an X dimension as small as 0.0025 inch. More complex logos are also being printed to take advantage of the availability of the higher resolution print heads.
An application where high density bar codes are being used is in the manufacturing of complex circuit boards. Common label sizes for circuit boards range from 5 mm to 15 mm in length. Despite the small size of such bar codes, they must be able to be identifiable.
Conventional thermal transfer ribbons do not image single isolated dots or fine line bar codes (lines with an X dimension of 1 dot) well. Conventional ribbons have heat-meltable ink layers composed primarily of wax or of both wax and resin. The wax-type thermal transfer ribbons can produce bar code images with sharp vertical lines, but the images have poor scratch resistance. The resin-type thermal transfer ribbons produce images with high scratch resistance, but poor fine line print quality.
In view of the foregoing, it is an object of the present invention to provide a thermal transfer ribbon that is capable of printing a highly scratch resistant, high density and high resolution bar code image.